y=mx + c
first find 'm'
m=y2-y1/x2-x1
m= 21-15/7-3
m=6/4 or 3/2
then substitute values in from either points
y=mx+c
15=3/2 * 3 + c
15=9/2 + c
15-9/2=c
11/5=c
therefore the equation must be y=3/2 x 11.5
note: you substitute with either (3,15) or (7,21) you will get the same answer either way.